Worth noting: The bug didn’t go so far as to make the would-be blocked individual your friend (even if they were your friend prior to the block), so anything an affected individual might’ve posted to a friends-only audience should have remained private. It would, however, allow a blocked user to do things like contact you on Messenger, or try to re-add you as a friend.

While Facebook isn’t saying much about what caused the bug, they’re sending out a notification (pictured above) to anyone they believe was affected.

Update — Facebook has shared some more details about the cause of the bug with TechCrunch’s Josh Constine via Twitter:

Hey Josh, it’s always hard to find the right level of technical detail to put in a blog post like this. More context on what caused the bug: most visible user data on FB is stored in pairs called “associations,” which control what posts people see and the actions they can take.